Due to the limited availability and rising cost of petroleum, it is becoming increasingly desirable to develop nonpetroleum energy sources and to maintain a stable supply thereof. The more effective utilization of coal, which would greatly reduce the dependence of the world economy on petroleum, is now being enthusiastically reconsidered. However, since coal is a solid, its use presents transportation and handling difficulties not encountered with petroleum.
In order to facilitate handling and use of coal, coal-water slurries ("CWS"), consisting of fine particles of coal dispersed in water, have been developed. The amount of water necessary to form a pumpable slurry varies with the surface characteristics and extent of comminution of the coal. Without additives, a coal slurry made up of particles, most of which will pass through a 200 mesh Tyler sieve can only contain from about 40 to 45 weight percent coal. When the coal content of a slurry (without additives) is increased much beyond this level, the viscosity of the slurry rises markedly and the slurry becomes unpumpable. To disperse the coal particles in water, achieve a high solids content slurry, and maintain an acceptable slurry viscosity, it is necessary to use a surfactant to reduce the coal-water surface tension and thereby facilitate the wetting of particle surfaces. A suitable surfactant must also function to prevent the recombination of coal particles into large clusters. Also, a suitable surfactant must retain its dispersing function during and following slurry preparation and handling conditions involving high shear, such as comminution, mixing and pumping. See, for example, U.S. Pat. Nos. 4,302,212, Yamamura et al., issued Nov. 24, 1981; 4,304,572, Wiese et al., issued Dec. 8, 1981; and 4,104,035, Cole et al., issued Aug. 1, 1978; 4,282,006, Funk, issued Aug. 4, 1981; and 4,358,293, Mark, issued Nov. 9, 1982, all incorporated by reference herein. In general anionic surfactants, and in particular the sulfonates, have been recognized as the preferred class of surfactants for coal slurries.
U.S. Pat. No. 4,472,170, Hellyer, issued Sept. 18, 1984, and incorporated by reference herein, discloses nonionic alkyl glycosides as highly desirable surfactants for aqueous coal slurries.
Since surfactants add to the cost of the slurry without providing any significant increase in fuel value, it is desirable to employ surfactants which are relatively inexpensive and highly efficient; i.e., those which can be used at low levels and still provide low vicosity, easily pumpable slurries. Also, since many anionic surfactants contain elements such as sulfur, phosphorus or nitrogen, it is desirable to use these surfactants at the lowest possible levels in order to minimize air pollution by sulfur, phosphorus or nitrogen combustion products when the slurry is burned. Metallic cations (e.g., Na or K) in anionic surfactants produce ash and can contribute to slagging and fouling in boilers when the slurry is burned. Thus, low usage level of anionic surfactant in the slurry is also desirable in order to minimize ash formation.
The object of the present invention is to provide superior surfactants for use as dispersants in coal-water slurries. A further objective is to provide anionic sulfate surfactants which are highly efficient dispersants for coal-water slurries, thereby making it possible to use the surfactants at low levels and thus minimize introduction of air pollution-producing, or ash-producing, elements.